Allow customized coveragerc for Python coverage

### Description of the feature request:

For python coverage collection, `.coveragerc` is hardcoded in https://github.com/bazelbuild/bazel/blame/d435c6dd6e977a5c3ea1bc726557a9321948a317/tools/python/python_bootstrap_template.txt#L393-L397. To provide configurability, we'd like to introduce custom `.coveragerc` to set configurations including https://coverage.readthedocs.io/en/stable/excluding.html#advanced-exclusion.

### Which category does this issue belong to?

Configurability, Python Rules

### What underlying problem are you trying to solve with this feature?

Allow coverage configurations. In particular, we're interested in excluding import statements being collected as executable lines in coverage reports.

### Which operating system are you running Bazel on?

macOS, Linux

### What is the output of `bazel info release`?

release 6.3.0

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse master; git rev-parse HEAD` ?

_No response_

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

まず tools/python/python_bootstrap_template.txt の393～397行目付近を読み、.coveragerc のパスがハードコードされている箇所を確認して、macOS と Linux で Python のカバレッジがどのように収集されるかを追跡します。完了の条件は、ユーザーがカスタムの .coveragerc を指定し、import 文を含むカバレッジの除外を設定でき、デフォルトの動作が使用できなくならないことです。

索引モデルが issue の本文から書いたものです。